The present invention relates generally to memory devices and, more particularly, to a method for controlling current fluctuation during read and program operations.
Conventional memory architectures employ word lines and bit lines to access the memory cells defined by the memory architecture. Three dimensional (3D) memory structures include several memory layers that are stacked one on top of the other. FIG. 1 illustrates a 3 dimensional memory structure. The memory structure includes word lines 12 and bit lines 10. The word lines (WL) and the bit lines (BL) are connected to word line and bit line decoder transistors 14. The bit lines 10 are connected to programmable diodes 16. During read and program operations the decoder transistors 14 select the bit line and the read line corresponding to a memory cell to be accessed (not shown). In a 3D memory structure as shown in FIG. 1, the layer decoder transistors are employed in both the WL and the BL for the selection layers. Therefore, the equivalent circuit can be treated as two MOSFETs connected by a programmable diode 16 and a series resistor (not shown). Generally, the decoder transistors 14 act as switches. When the transistor is turned on, a high gate voltage (VG) is applied so that the MOSFET will be in a low-resistance status. As a result, the MOSFET will act like a switch.
Generally, a read or program operation is carried out by applying constant high voltages to the selected WL and BL. However, the diode-current during the read operation varies exponentially with the voltage. Hence, a small device variation or the loading effect due to the series resistance of WL and BL may cause large variations in the current during the read operation. In such situations, the voltage required for the read operation must be increased to ensure that the current through all the programmable diodes are larger than a predetermined current required for the read operation. This may cause some of the diodes to undergo large stresses, due to the high current, during read and program operations. This stress can cause reliability problems as the programmable diode has a tendency to go into an un-programmed state where the diode acts as an open-circuit.
FIGS. 2 and 3 graphically illustrates the relatively large variations in current during read operations when there are small changes in voltage. FIG. 2 is a graph showing the overall current applied during read operations. The overall current is shown on the y-axis while the applied voltage is shown on the x-axis. As can be seen, there is a two orders of magnitude difference in the overall current due to loading effect and diode variation, as shown in region 22.
FIG. 3 is a graph further illustrating a condition where there is a large variation in diode current as a result of small fluctuations in voltage. In FIG. 3, the voltage during the read operation is represented on the x-axis and the current during the read operation is represented on the y-axis. In this case, the minimum required read current is indicated by line 32. According to FIG. 3, the voltage required for the minimum current is VR. The choice of VR is to ensure that all diodes can be correctly read. However, in this case for the same VR value, the current required by some of the diodes during the read operation is illustrated by current values corresponding to lines 34 and 36 so that the current through the diodes during the read operation remains larger than a minimum required value. The current values corresponding to lines 34 and 36 are relatively high compared to the current value at line 32. As described above, the diodes with such high current values are likely to undergo large stress.
Similarly, the ON state characteristics of the programmable diode are strongly dependent on the current required for the program operation. Therefore, the current on the diodes during the program operation must be kept at a constant in order to prevent device-characteristic variation.
In the past a constant voltage has been applied to both the word line and the bit line during the read and program operations. However, this approach was not proven to be effective, as it did not address the issue of large current fluctuations in the diodes during read operations.
In light of the foregoing, there is a need to minimize fluctuations in current value during read and program operations.